(a) Field of the Invention
The present invention relates to a method for controlling a skip down shift of an automatic transmission including a clutch to one-way clutch mechanism, a primary shift portion and a secondary shift portion. More particularly, the present invention relates to a method for controlling a skip down shift of an automatic transmission by synchronizing the secondary shift portion to a target shift-speed through an engine torque reduction control.
(b) Background
Generally, in an automatic transmission, a target shift-speed is determined by a map table of shift patterns predetermined corresponding to an opening change of a throttle valve and a vehicle speed.
Then, operational elements of a mechanism including an on-coming clutch and an off-going clutch are operated by a duty control of hydraulic pressure.
Therefore, a shift speed to the target shift-speed is automatically realized and driving convenience is supplied.
A control logic of a down shift before a vehicle stops is predetermined in the automatic transmission.
An object for controlling the down shift before the vehicle stops is to improve accelerating performance and drivability by increasing a gear ratio when the vehicle is re-accelerated in a case that the vehicle slows down.
A shift control before a vehicle stops of an automatic transmission having a clutch to clutch mechanism includes a power-on down shift and a power-off down shift corresponding to a power state.
A shift control before a vehicle stops of an automatic transmission having a clutch to one-way clutch mechanism includes only a power-on down shift.
Generally, an automatic transmission having the clutch to one-way clutch mechanism can operate the power-on down shift control without additional elements in a low speed area, and so shift feel and drivability are satisfied.
However, if a multiple skip down shift occurs before the vehicle stops in a high speed area, because the automatic transmission is seriously affected by a deviation of a control system and an engine idle rotation speed, a problem occurs that shift feel is deteriorated.
In addition, if a tip-in occurs during the skip down shift, a problem occurs that serious shift shock occurs.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.